"Unfortunately, the g++ compiler did compile this piece of code because 'j1' is defined as a Bessel function in 'cmath'."

No, the "std" name-space does not have a function called "j1" -- unless your compiler has defined a function call "j1" somewhere inside its implementation. Sometimes, the compiler is able to define functions that are accessible in every scope; "__builtin_...( )" is an example of one of those functions.